III-V Nanowire Array Solar Cells: Optical and Electrical Modelling

This thesis describes optical and electrical modelling of vertically oriented III-V semiconductor nanowire array solar cells (NWASCs).In the optical studies, three-dimensional electromagnetic modelling was carried out with the scattering matrix method and the finite element method. Shockley–Queisser detailed balance analysis was first applied to a dual-junction NWASC in which both junctions were within the nanowires. A limit for the solar cell efficiency was calculated for various nanowire array geometries and materials. The dimensions giving the highest efficiency limit were determined by applying an optimization method that does not rely on the use of derivatives. The optical response of a nanowire array subcell on top of a planar silicon subcell was thenconsidered. For the optimized geometry with approximately 15 nanowires per square μm, a conformal top contact of transparent conductive oxide led to large absorption losses. It was found that a scheme with a planarized top contact reduced these losses. Furthermore, the benefit of using anti-reflection coatings on top of the planar top contact and on the substrate surface between thenanowires was demonstrated. With such coatings, the number of reflected photons could be reduced by 80%.The full opto-electrical response of a single-junction InP NWASC was also studied by combining optical modelling with drift-diffusion-based electrical modelling. The spatially resolved internal quantum efficiency was used as a tool to understand the position-dependent electron–hole splitting efficiency for various designs of the p-i-n junction within the nanowires. Importantly, itwas found that contact leakage is an important loss mechanism. A high-bandgap semiconductor, GaP, is suggested as a material for the n-doped top segment, which increases the total electron–hole splitting efficiency to almost 100%. In addition to this top selective contact, the GaP substrate can be used as a p-doped bottom-selective contact. With the application of both selectivecontacts, it is possible to improve the open-circuit voltage of InP NWASCs to the Shockley–Queisser (S–Q) open-circuit voltage of 1.02 V.Apart from light absorption and solar cell efficiency calculations, a light-emission model for periodic nanostructures was developed by combining Green’s dyads and the drift-diffusion model. This model takes the Purcell effect into consideration, which makes it possible to calculate both near-field and far-field emission powers